Within the blast area there is a small central crater composed of blocky lava covered by a thin layer of ash. Surrounding this crater is an area of'the piedmont plain covered by ash generally over 30 cm. in depth. In addition, a section of the plain is covered by lahar deposits of considerable depth. A few steam vents are active but devoid of vegetation. No sign of sulphur was noted although sulphurous fumes had been reported during the eruption by Baker. In addition, there is a small section of the steep Goropu mountains within the blast area, but this area was not visited. The vegetation ranges from a forest community to an open community of scattered herbs, and these vegetation types are closely correlated with the edaphic conditions.
Several distinct communities are found on the crater walls. Growing on the greatest part of the area is an open community dominated by the grasses Saccharum spontaneum and Imperata cylindrica, which is very similar to the grass communities found on Avalanche Valley 2 km'. from the dome of Mt. Lamington. The deposits on which these two communities are growing are very similar in texture. On the lower slopes of the crater wall the open community is invaded by a few species of small trees; Euroschinuspapuanus, Terminalia sp., 'gorli' (On),Pipturus sp., 'kuraika' (Or) and Casuarinapapuana S..Moore. In small depressions these species form a low woodland with a dense ground layer of ferns and grasses. Other tree species are uncommon. A second woodland up to 8 m. in height is found on the many small
TOPSOIL COMPLETELY REMOVED
LOWER ~UBSOIL EXPOSED
INCREASING DEPTH OF ASH
Fig. 2. Environmental distribution of communities. Mt. Lamington blast area.
dry gullies which drain radially down the crater walls. The soils in these gullies consist mainly of sand and gravel but finer fractions are more abundant than under the surrounding open grassland and remain moist for a much longer period. This woodland is dominated by Casuarina papuana with 80 per cent cover. Other species present are Euroschinus paj!manus, Terminalia sp., 'gorli' (On),Octomeles sumatrana, Ficus pungens and Pipturus sp. The ground layer is very sparse but climbers are frequent.
The vegetation on the almost flat lahar deposits consists of tall grassland sur- rounding scattered bare areas. These bare areas cover 20 per cent of the deposit and have a surface crust 5 cm. thick. The grassland, 4 m . in height, is growing on coarse s a n d a n d c o n s i s t s o f a n a l m o s t p u r e s t a n d o f S a c c h a r u m s p o n t a n e u m , o t h e r g r a s s e s , small trees and twining plants being rarely found. ' .